That means that for this type of work, smart contracts are a godsend.
Check this website:
http://clientsfromhell.net/ Every single one of these horror stories can be avoided by smart contracts. That's because smart contracts give both parties the incentive to uphold their part of the bargain to the best of their abilities. It's not about a 'he said she said' issue with a broken guitar or a box filled with phonebooks.
It's about "Hey I like the site so far but our contract also says that the member profile section has to be finalised. If you finish it this week we can both release each other's eskrow, if you think you're not getting it done we can just pay for the finished part for 80% and make a new contract for the member profile section part."
These deals need a lot of communication and this can only truly be done when both parties have leverage on each other.
Does the wallet allow a contract to be closed out at an agreed upon lower price? The point you raise here makes it clear that that's an important option for when a contract or an order can only be partially fulfilled.
When both parties agree on the situation, as in your example, and both negotiate in good faith, it should work fine. I guess escrow also pressures both parties to meet in the middle when they disagree, even if one party is objectively wrong but (because of ignorance or stupidity, as in many of the examples on that site) will never be convinced that they're wrong. That's often what would happen in real life anyway I assume.